As is pointed out in "The Contribution of Fluid-Bed Technology to Energy Saving and Environmental Protection", Botterill and Virr, Applied Energy (3) (1977), fluid-bed solid fuel combustors offer certain advantages over conventional boilers. For example, a conventional power station boiler burns coal in about 40,000 times its own volume of air. Heat transfer conditions are poor--e.g. the excessive volume flow of gas removes too much heat. Temperature conditions within the combustion chamber vary substantially, with flame temperatures of about 2800.degree. F., and this leads to regions of high corrosion. Moreover, flame is a relatively inefficient heat radiator.
The fluidized bed combustor on the other hand overcomes many of these problems. By employing hot inert solid particles (e.g. ash or sand) in the bed, heat is supplied to the fuel particles to raise them to combustion temperature. Adequate combustion oxygen is supplied through nozzles in the bed but the overall ratio of air volume to bed volume is relatively low--e.g. 1:160 as opposed to 1:40,000 for conventional boilers. The high heat capacity of the inert material, combined with the fluid or highly agitated nature of the bed, prevents the burning fuel from reaching excessive temperatures and helps maintain uniform temperatures throughout the bed. The hot inert material is, of course, an efficient means by which to transfer heat to the heat extraction tubes which pass through the bed.
The fluid bed boiler has also generated increased interest as the cost of fuels has increased and as environmental standards have become paramount. In this regard, the fluid bed combustor permits the use of high sulfur coal, which although lower in price than most other fuels, normally is either environmentally unacceptible or requires expensive pollution control apparatus. Thus, there may be added to the inert bed material crushed limestone or other known materials which convert sulfur oxides to sulfate. The sulfate is retained within the bed and may be disposed of by known means.
Most fluid bed combustors developed to date have employed horizontally oriented heat extractor tubes in the bed. Such configurations, while offering to varying degrees the advantages discussed above, are unsuited for natural circulation of the fluid within the tubes. They are also subject to tube erosion in certain temperature regimes and limited in turndown or power output control since heat transfer surface (i.e. the relative length of heat extractor tubes immersed within the bed at any given time) will not vary uniformly or directly with changes in bed depth. Horizontal tubes also generally experience lower heat transfer rates than vertical tubes.
Arrangements involving vertical bed tubes have been limited to waterwalls around the periphery of the combustion chamber and arrangements, such as disclosed in U.S. Pat. No. 3,983,927, in which the tubes are mostly vertical but do not extend through the distributor plate. In the former case, the heat extraction potential of the bed is not fully exploited; in the latter case, natural circulation is unattainable.